Experimental and computational studies of a multi-electron donor-acceptor ligand containing the thiazolo[5,4-d]thiazole core and its incorporation into a metal-organic framework

Abstract

A ligand containing the thiazolo[5,4-d]thiazole (TzTz) core (acceptor) with terminal triarylamine moieties (donors), N,N'-(thiazolo[5,4-d]thiazole-2,5-diylbis(4,1-phenylene))bis(N-(pyridine-4-yl)pyridin-4-amine (1), was designed as a donor-acceptor system for incorporation into electronically active metal-organic frameworks (MOFs). The capacity for the ligand to undergo multiple sequential oxidation and reduction processes was examined using UV/Vis-near-infrared spectroelectrochemistry (UV/Vis-NIR SEC) in combination with DFT calculations. The delocalized nature of the highest occupied molecular orbital (HOMO) was found to inhibit charge-transfer interactions between the terminal triarylamine moieties upon oxidation, whereas radical species localized on the TzTz core were formed upon reduction. Conversion of 1 to diamagnetic 2+ and 4+ species resulted in marked changes in the emission spectra. Incorporation of this highly delocalized multi-electron donor-acceptor ligand into a new two-dimensional MOF, [Zn(NO3 )2 (1)] (2), resulted in an inhibition of the oxidation processes, but retention of the reduction capability of 1. Changes in the electrochemistry of 1 upon integration into 2 are broadly consistent with the geometric and electronic constraints enforced by ligation.

Keywords: density functional calculations; donor-acceptor systems; electron transfer; metal-organic frameworks; spectroelectrochemistry.